In recent years, since the amount of information processed by a computer has been increasing, there has been a strong demand for an increase in the capacity of a hard disk drive as an auxiliary storage apparatus. Moreover, hard disk drives have come to be mounted inside domestic electric products, and the demand for downsizing the hard disk drive and increasing the capacity becomes even stronger.
In a longitudinal magnetic recording system conventionally used for a hard disk drive, since magnetizations recorded in the medium are adjacent to each other and face in opposite directions, in order to increase the linear recording density it is necessary not only to increase the coercivity of the recording layer but also to decrease the film thickness. However, a problem arises that the write-ability of the write head becomes insufficient with increasing coercivity of the recording layer, and a problem arises that recorded information is lost by thermal fluctuation with decreasing thickness of the recording layer; thereby, it becomes difficult to improve the areal recording density using a longitudinal magnetic recording system. In order to solve these problems, there has been active development of a hard disk drive using a perpendicular magnetic recording system. The perpendicular magnetic recording system is a method whereby a recorded bit is formed such that magnetization of the recording medium is perpendicular to the medium face and the magnetizations in adjacent recorded bits are antiparallel, thus making it possible to decrease the medium noise because a demagnetization field in the magnetic transition region is small compared with a longitudinal magnetic recording system and it is possible to stably maintain the recorded magnetization during high density recording. Moreover, a method has been proposed where a double-layer perpendicular magnetic recording medium which includes a soft-magnetic underlayer working as a return-path of flux between the perpendicular magnetic recording medium and the substrate is combined with a single-pole-type head (a so-called SPT head). A magnetic head (a so-called TS head) has been further proposed where a magnetic shield is provided through a non-magnetic gap layer at the trailing side of a main pole in order to improve the write-field gradient.
A structure has been proposed for a magnetic recording layer of perpendicular magnetic recording media where magnetic grains are magnetically decoupled by segregating a non-magnetic compound such as an oxide and a nitride around the magnetic grains (a so-called granular structure). For example, “Role of Oxygen Incorporation in Co—Cr—Pt—Si—O Perpendicular Magnetic Recording Media” IEEE Transactions on Magnetics, Vol. 40, No. 4, July 2004, pp. 2498-2500, discloses a method for forming a recording layer having a granular structure by using a composite target containing a CoCrPt alloy and SiO2 and by using a DC magnetron sputtering technique in an argon-oxygen mixed gas atmosphere.
As a means for lowering the noise of a medium and improving the SNR, Japanese Patent Publication No. 2006-302426 (“Patent Document 1”) discloses a magnetic recording layer having a granular structure which includes Co, Cr, Pt, Si, and O, the magnetic grain size practically constant in the film thickness direction, and a region where the interface side with the intermediate layer contains more oxygen than the surface layer. Japanese Patent Publication No. 2004-025943 (“Patent Document 2”) discloses that a recording layer is formed of two or more magnetic layers having different oxide contents, crystal grains in the lowermost layer of the recording layer being made minute by making the oxide content of the magnetic layer largest at the side closest to the substrate, and a magnetic layer is laminated thereon where the crystal grains are larger than the crystal grains of the lowermost layer. Japanese Patent Publication No. 2004-310910 (“Patent Document 3”) discloses that a Co—Cr alloy layer which does not include oxide is laminated over a recording layer having a granular structure where an oxide is segregated to the grain boundaries. In addition, Patent Publication No. 2006-309919 (“Patent Document 4”) discloses a technique where a magnetic layer is formed of two or more layers having a granular structure and the lower magnetic layer has a greater non-magnetic and immiscible atomic concentration than the upper magnetic layer.